It is known in the oilfield drilling industry to take measurements near a drill bit to assist in evaluating and locating a well, based on geological, geometrical, and environmental parameters. This information may be stored in memory for later retrieval and/or telemetered to surface in near real-time in a technique known as Measurement While Drilling (“MWD”).
A common method of MWD transmission to surface uses a low frequency Electro-Magnetic (“EM”) signal created by applying an alternating voltage across an insulating joint in the drill string, thereby inducing a current to flow through the earth formation and back to surface where it is detected by sensitive receivers. The insulating joint or “gap joint” is often formed as part of a component of the drill string known as the “gap sub”, wherein the “gap” refers to a length of non-conductive material interposed between two conductive metal tubular components and the term “sub” refers to a short length of drill collar.
In some prior art examples of gap sub designs, there are disclosed a rigid insulated antenna connection which traverses the length of the gap sub along the axis of the bore. This antenna and any additional fixturing to hold it partially obstructs the bore, precluding further use of the bore to conduct logging tools or other equipment there-through.
In other prior art gap sub designs, the presence of an antenna is not specifically disclosed, and instead, it is known to use a probe containing an insulated signal conductor along with other electronics required to produce the transmission signal; the conductor traverses the length of the gap sub along the axis of the bore thereby making electrical contact on both sides of the gap joint to effect signal transmission. While it is often economical and convenient to contain the electronics, batteries, and sensors in a probe centered along the gap sub axis (and thus has become the industry standard), the use of a probe obstructs the bore, and prevents the bore for being used to conduct additional equipment there-through.
Sub-surface signal transmitting apparatus are known in other applications, such as post-well drilling formation evaluation. However, such apparatus not being used in drilling applications do not have the necessary structural properties for use in a drill string. For example, such apparatus typically have cabling and batteries located in exposed locations on the outside of the apparatus which thus would be exposed to damage when used in a drilling application.
It is therefore desirable to provide to an EM telemetry assembly for use in a drill string that provides a solution to at least some of the deficiencies in the prior art.